In the cause of improving the diagnosis and treatment of cancer numerous attempts have been made to target imaging agents, e.g. radioactive isotopes, and therapeutic reagents onto tumors in vivo using anti-tumor monoclonal antibodies, Mabs, see for example published International Applications WO 89/00583 and WO 90/09197. Because of the size of the average Mab, diffusion rates of, for example, a radio-labelled antibody to the site of a tumor are very low, and the same applies to conjugates formed from a cytotoxic reagent and an anti-tumor antibody.
Tumor imaging and therapy using Mabs is therefore inherently a slow process, often taking several hours, whereas ideally one would like to have the process complete in minutes, rather than hours, particularly in the case of tumor imaging and diagnosis.
In J. Immunol., 145, No, 1, 59-67, 1990, Shimizu et al have reported on the co-stimulation of proliferative responses of resting CD4U.sup.+ T-cells by the interaction of the extracellular matrix (ECM) proteins fibronectin (FN) and laminin (LN) with the VLA integrins VLA-4 and VLA-5 (in the case of fibronectin) or VLA-6 (in the case of laminin) expressed by resting human T-lymphocytes, and have shown inter alia that the 12-amino acid peptide ##STR1## is an effective T-cell adhesion inhibitor and an effective blocking agent for OKT3/FN T-cell proliferation, but does not go beyond a mere investigation into the role of cell adhesion molecules (CAMs) in T-cell recognition and activation. Thus, Shimizu et al suggest no practical outcome or industrial utility resulting from their investigations.
In J. Biol. Chem., 266, No. 6, 3579-3585, 1991, Mould et al have reported their studies of the inhibition of the interaction of the integrin heterodimer .alpha..sub.4 .beta..sub.1 with the CS1 and CS5 sites in the IIICS region of fibronectin, which interaction is believed to play an important role in melanoma cell adhesion, and have shown that the tripeptide X-asp-Y, where X is glycine, leucine or glutamic acid, and Y is serine or valine, represents a minimum recognition sequence within the CS1 site of the fibronectin for the integrin .alpha..sub.4 .beta..sub.1. Inter alia, those studies utilised synthetic CS1 and KKT-CS1-VQK peptides, viz: ##STR2## but again no consequential commercial utility is suggested for such LDV (leu-asp-val) containing peptides.
Subsequent studies of similar kind confirming the significance of the LDV triplet as the minimum recognition site for the .alpha..sub.4 .beta..sub.1 integrin, as opposed to the RGDS sequence which constitutes the minimum recognition site for the .alpha..sub.5 .beta..sub.1 integrin, those two minimum recognition sites occurring in different domains of the fibronectin (FN) molecule, viz. the alternatively spliced type III connecting segment (IIICS) and the central cell binding domain respectively, are reported by Komoriya et al in J. Biol. Chem., 266, No. 23, 15075-15079, 1991, but published after the present priority date.
Earlier studies into the cell binding activity of fibronectin are reported by Pierschbacher and Ruoslahti (Nature, 309, 30-33, 1984) and by Kloczewiak et al in Biochemistry 28, 2915-2919, 1989. Pierschbacher et al recognize the importance of the RGDS sequence as a recognition site in the cell adhesion activity of fibronectin, but report that they were unable to obtain any inhibition of cell adhesion using either soluble fibronectin or by peptides not containing the RGDS sequence. Similarly Kloczewiak et al, investigating the essential role of the terminal region of the fibrinogen .gamma.-chain in the interaction of human fibrinogen with activated platelets, showed that the synthetic dodecapaptide: ##STR3## being an analogue of the .gamma.400-411 FN residue, and containing the LDV triplet, achieved 50% inhibition of fibrinogen binding to activated platelets, but only at concentrations greater than 500 .mu.M.
In contrast to both Shimizu et al and Mould et al, EP-A-0428266 identifies as potential anti-cancer agents, peptides based on the functional domains of fibronectin, more particularly the cell binding domains and the heparin binding domains, amino acid residues 1239-1515 and 1690-1960 respectively (EP-A-0428266, SEQ ID Nos. 2 and 3). According to EP-A-0428266, these fibronectin fragments show anti-cancer activity in mice by inhibiting metastasis, and may be used singly or in the form of a chimeric peptide comprising both sequences linked by a linker, e.g. a methionine residue, EP-A-0428266 SEQ ID No. 4. In either event, the peptide fragment is of substantial size: 277 amino acid residue in the case of the cell binding domain FN fragment, and 271 in the case of the heparin binding FN fragment, making a total of 549 amino acid residues in the chimeric peptide. Such fragments are therefore still of substantial size compared to the oligopeptides used in the present invention, and can be expected still to show many of the disadvantages of the Mab based anti-cancer agents, namely low diffusion and clearance rates. Not only that, but none of the FN fragments disclosed as anti-cancer agents in EP-A-0428266 contain the amino acid triplet which is characteristic of the oligopeptides used in accordance with the present invention, that is to say the leu-asp-val (LDV) triplet.
More recently, in published International Patent Application WO 90/15858, the present inventor has disclosed novel peptides comprising an RGD sequence (i.e. the sequence: arg-gly-asp) primarily for in vivo thrombus imaging, but also potentially useful for in vivo tumor imaging and therapy, and capable of binding to tumors in vivo via RGD binding sites on the surface of the tumor. In the case of thrombus imaging the peptide binds to RGD recognition and binding sites in the GPIIb/IIIa (glycoprotein-fibrinogen receptor) complex present on the membrane surface of activated platelets, and which contain the fibrinogen binding domains and which are involved in the aggregation of the activated platelets to form the thrombus.